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<p>这是在变量以空初始化器构造时进行的初始化。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">T</span><code><b>();</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>new</b></code> <span class="t-spar">T</span> <code><b>();</b></code>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">Class</span><code><b>::</b></code><span class="t-spar">Class</span><code><b>(</b></code><span class="t-spar">...</span><code><b>)</b></code> <code><b>:</b></code> <span class="t-spar">member</span><code><b>()</b></code> <code><b>{</b></code> <span class="t-spar">...</span> <code><b>}</b></code>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">T</span> <span class="t-spar">object</span> <code><b>{};</b></code>
</td>
<td> (4)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">T</span><code><b>{};</b></code>
</td>
<td> (5)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>new</b></code> <span class="t-spar">T</span> <code><b>{};</b></code>
</td>
<td> (6)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">Class</span><code><b>::</b></code><span class="t-spar">Class</span><code><b>(</b></code><span class="t-spar">...</span><code><b>)</b></code> <code><b>:</b></code> <span class="t-spar">member</span><code><b>{} </b></code> <code><b>{</b></code> <span class="t-spar">...</span> <code><b>}</b></code>
</td>
<td> (7)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p>以下场合进行值初始化：
</p>
<div class="t-li1"><span class="t-li">1,5)</span> 当以空的括号<span class="t-rev-inl t-since-cxx11"><span>或花括号</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>对组成的初始化器创建无名临时对象时；</div>
<div class="t-li1"><span class="t-li">2,6)</span> 当 <a href="new.html" title="cpp/language/new">new 表达式</a>以空的括号<span class="t-rev-inl t-since-cxx11"><span>或花括号</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>对组成的初始化器创建具有动态存储期的对象时；</div>
<div class="t-li1"><span class="t-li">3,7)</span> 当使用以空的括号<span class="t-rev-inl t-since-cxx11"><span>或花括号</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>对组成的<a href="constructor.html" title="cpp/language/initializer list">成员初始化器</a>初始化非静态数据成员或基类时；</div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<div class="t-li1"><span class="t-li">4)</span> 当以由空花括号对组成的初始化器声明具名变量（自动、静态或线程局部）时。</div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<p>所有情况下，若使用空花括号对 <code>{} </code> 且 <code>T</code> 是<i>聚合类型</i>，则进行<a href="aggregate_initialization.html" title="cpp/language/aggregate initialization">聚合初始化</a>而非值初始化。
</p><p>若 <code>T</code> 是没有默认构造函数但带有接受 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a></span></span> 的构造函数的类类型，则进行<a href="list_initialization.html" title="cpp/language/list initialization">列表初始化</a>。
</p><p><br>
值初始化的效果是：
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx11"><td>
<div class="t-li1"><span class="t-li">1)</span> 若 <code>T</code> 是有至少一个用户提供的任意种类的构造函数的类类型，则调用<a href="default_constructor.html" title="cpp/language/default constructor">默认构造函数</a>；</div>
<div class="t-li1"><span class="t-li">2)</span> 若 <code>T</code> 是没有任何用户提供的构造函数的非联合体类类型，则值初始化 T 的每个非静态数据成员与基类组分；
</div></td>
<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td></tr>
<tr class="t-rev t-since-cxx11"><td>
<div class="t-li1"><span class="t-li">1)</span> 若 <code>T</code> 是没有默认构造函数，或拥有用户提供的或被删除的默认构造函数的类类型，则<a href="default_initialization.html" title="cpp/language/default initialization">默认初始化</a>对象；</div>
<div class="t-li1"><span class="t-li">2)</span> 若 <code>T</code> 是拥有默认构造函数的类类型，而默认构造函数既非用户提供亦未被删除（即它可以是拥有隐式定义的或默认化的默认构造函数的类），则<a href="zero_initialization.html" title="cpp/language/zero initialization">零初始化</a>对象，然后若其拥有非平凡的默认构造函数，则<a href="default_initialization.html" title="cpp/language/default initialization">默认初始化</a>它；
</div></td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<div class="t-li1"><span class="t-li">3)</span> 若 <code>T</code> 是数组类型，则值初始化数组的每个元素；</div>
<div class="t-li1"><span class="t-li">4)</span> 否则，零初始化对象。</div>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>若构造函数是用户声明的，且未在其首个声明上显式默认化，则它是<i>用户提供</i>的。
</p><p>语法 <span class="t-c"><span class="mw-geshi cpp source-cpp">T object<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span> 不初始化对象；它声明一个不接受参数并返回 <code>T</code> 的函数。在 C++11 前值初始化一个具名变量的方式是 <span class="t-c"><span class="mw-geshi cpp source-cpp">T object <span class="sy1">=</span> T<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span>，它值初始化一个临时量然后复制初始化该对象：多数编译器在此情况下能<a href="copy_elision.html" title="cpp/language/copy elision">优化掉复制</a>。
</p><p>在 C++03（引入了值初始化）之前的 C++98 中，表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">new T<span class="br0">(</span><span class="br0">)</span></span></span> 被归类为默认初始化并指定进行零初始化。
</p><p>引用不能被值初始化。
</p><p>如<a href="explicit_cast.html" title="cpp/language/explicit cast">函数式转型</a>中所述，语法 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="br0">(</span><span class="br0">)</span></span></span> <span class="t-v">(1)</span> 对于数组禁止，但允许 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="br0">{</span><span class="br0">}</span></span></span> <span class="t-v">(5)</span>。
</p><p>所有标准容器（<span class="t-lc"><a href="../container/vector.html" title="cpp/container/vector">std::vector</a></span>、<span class="t-lc"><a href="../container/list.html" title="cpp/container/list">std::list</a></span> 等），在以单个 <code>size_type</code> 实参进行构造或由对 <span class="t-c"><span class="mw-geshi cpp source-cpp">resize<span class="br0">(</span><span class="br0">)</span></span></span> 的调用而增长时，值初始化其各个元素。
</p><p>从 C++11 起，对没有用户提供的构造函数而拥有类类型成员的类进行值初始化，其中成员的类拥有用户提供的构造函数，会在调用成员的构造函数前对成员清零：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A
<span class="br0">{</span>
    <span class="kw4">int</span> i<span class="sy4">;</span>
    A<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// 用户提供的默认构造函数，不初始化 i</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> B <span class="br0">{</span> A a<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 隐式定义的默认构造函数</span>
 
<a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> B<span class="br0">(</span><span class="br0">)</span>.<span class="me1">a</span>.<span class="me1">i</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// 值初始化 B 临时量</span>
                              <span class="co1">// C++03 中令 b.a.i 为未初始化</span>
                              <span class="co1">// C++11 中设 b.a.i 为零</span>
<span class="co1">// （注意 C++11 中 B{}.a.i 保留 b.a.i 为未初始化，但因为不同原因：</span>
<span class="co1">// 在 DR1301 后的 C++11 中，B{} 是聚合初始化，它值初始化拥有用户提供构造函数的 A）</span></pre></div></div>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;vector&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw1">struct</span> T1
<span class="br0">{</span>
    <span class="kw4">int</span> mem1<span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> mem2<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 隐式默认构造函数</span>
 
<span class="kw1">struct</span> T2
<span class="br0">{</span>
    <span class="kw4">int</span> mem1<span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> mem2<span class="sy4">;</span>
    T2<span class="br0">(</span><span class="kw4">const</span> T2<span class="sy3">&amp;</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// 用户提供的复制构造函数</span>
<span class="br0">}</span><span class="sy4">;</span>                    <span class="co1">// 无默认构造函数</span>
 
<span class="kw1">struct</span> T3
<span class="br0">{</span>
    <span class="kw4">int</span> mem1<span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> mem2<span class="sy4">;</span>
    T3<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// 用户提供的默认构造函数</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 类 =&gt; 默认初始化，值为 ""</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">int</span> n<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>                <span class="co1">// 标量 =&gt; 零初始化，值为 0</span>
    <span class="kw4">double</span> f <span class="sy1">=</span> <span class="kw4">double</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// 标量 =&gt; 零初始化，值为 0.0</span>
    <span class="kw4">int</span><span class="sy2">*</span> a <span class="sy1">=</span> new <span class="kw4">int</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 数组 =&gt; 每个元素的值初始化</span>
                            <span class="co1">//          每个元素的值为 0</span>
    T1 t1<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>                <span class="co1">// 有隐式默认构造函数的类 =&gt;</span>
                            <span class="co1">//     t1.mem1 被零初始化，值为 0</span>
                            <span class="co1">//     t1.mem2 被默认初始化，值为 ""</span>
<span class="co1">//  T2 t2{};                // 错误：类无默认构造函数</span>
    T3 t3<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>                <span class="co1">// 有用户提供默认构造函数的类 =&gt;</span>
                            <span class="co1">//     t3.mem1 被默认初始化为不确定值</span>
                            <span class="co1">//     t3.mem2 被默认初始化，值为 ""</span>
    <a href="../container/vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> v<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// 值初始化每个元素</span>
                            <span class="co1">// 每个元素的值为 0</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> n <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> f <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> a<span class="br0">[</span><span class="nu0">9</span><span class="br0">]</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> v<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> t1.<span class="me1">mem1</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> t3.<span class="me1">mem1</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    delete<span class="br0">[</span><span class="br0">]</span> a<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">0 0 0 0 0
0 4199376</pre></div></div> 
</div>
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> 应用于
</th>
<th> 出版时的行为
</th>
<th> 正确行为
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue1301">CWG 1301</a>
</td>
<td> C++11
</td>
<td> 默认化的默认构造函数跳过构造前的零初始化
</td>
<td> 进行零初始化
</td></tr></table>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<ul><li> <a href="default_initialization.html" title="cpp/language/default initialization">默认初始化</a>
</li><li> <a href="direct_initialization.html" title="cpp/language/direct initialization">直接初始化</a>
</li><li> <a href="copy_initialization.html" title="cpp/language/copy initialization">复制初始化</a>
</li><li> <a href="list_initialization.html" title="cpp/language/list initialization">列表初始化</a>
</li></ul>

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